1. Field of the Invention
The present invention relates to asphalt pavement. More particularly, though not exclusively, the present invention relates to an apparatus and method for determining the quality of asphalt pavement in real time.
2. Problems in the Art
In order to rebuild and pave existing roads or highways that show signs of cracking and significant deterioration, or when building new roads or highways, it is important to effectively control the paving process. In the construction of roads, highways, or parking lots and the like using asphalt pavement, various factors affect the quality of the resulting asphalt surface. These factors include the proper mix of asphalt components and the proper compaction of the asphalt.
Hot asphalt mix has three components including aggregate, asphalt cement, and air voids. The force from a load on the pavement is transmitted through the pavement by interlocking contact between the aggregate particles. Friction at these contact points gives the asphalt pavement its stability. The asphalt cement binds the aggregate particles together after they have been compacted and the asphalt cooled. Air voids formed in the hot asphalt mix make up approximately 15% of the volume of the hot mix. Proper compaction (described below) reduces this percentage to about 5%.
After the hot asphalt mix is laid by the paver, it is compacted using one or more types of rollers. A first type of roller is a break down roller or vibratory roller. This is the first roller that is typically used and provides most of the compaction. A vibratory roller has one or two steel drums with rotating weights which vibrate the drums creating a dynamic force that adds to the dead weight of the roller and increases the compacting force. A second type of roller is a rubber tired roller or pneumatic roller which includes a number of rubber tires staggered to provide a complete coverage. The rubber tired roller kneads the surface and closes the pores formed on the top surface of the asphalt. A third type of roller is the finish roller which is used last to get rid of any creases formed by one of the previous rollers.
Many individuals are involved in producing a quality asphalt pavement (e.g., mix specifier, hot mix plant operator, quality assurance/quality control inspector, asphalt laying operator, and roller operator), but the roller operator's skill ultimately determines the final quality of the compacted mat, which directly relates to its density. Only a carefully planned rolling pattern gives the uniformity and desired density. The roller can achieve proper compaction only by monitoring the asphalt density in real time. An under-compacted asphalt mat is permeable to air and water, which shortens the pavement life. In addition, there will be too many air voids which makes the asphalt pavement less stable since the number of inter-particle contact points is reduced and it is more susceptible to freeze-thaw conditions. On the other hand, unnecessary over-compaction will crush the aggregate which will lead to a reduction of air void content, which, in turn, can make the pavement susceptible to permanent deformation. If the pavement is under-compacted or over-compacted, the paving contractor typically gets penalized. It is therefore very important to achieve the proper level of compaction.
Extensive effort has been devoted in the past to the nondestructive evaluation (NDE) of asphalt density characteristics. Nuclear density gauges, for example, have been used for several years to measure the bulk density of hot asphalt mixtures quickly and nondestructively. Some nuclear density gauges have even been attached to rollers to continuously measure density during the rolling operation. Troxler Electronic Laboratories, Inc., a manufacturer of such devices, has recently developed a new, surface moisture density gauge using a gamma ray backscattering approach that can be used in the field during the paving operation and provide results within seconds to minutes. These devices seem promising but have their limitations. For example, these devices require several minutes to obtain an accurate density measurement making it difficult to implement in real time on a continuous basis. Research has shown that there is significant variability of density test results between the nuclear density gauge and standard core sample approach. The variability of density tests is lower with core measurements than with nuclear density measurements. Because of this, some core samples need to be taken to ensure that an acceptable density is obtained. Furthermore, this technique requires measurements at discrete locations and does not readily lend itself to performing continuous density measurements on a moving roller. In addition, nuclear density gauges involve the use of ionizing radiation which requires safety precautions, certification of equipment, etc.
Ground-penetrating radar (GPR) is a promising NDE technique that has possibilities for measuring asphalt density in real time during the rolling operation. However, extended sensor calibrations are required for producing meaningful results. Ground-penetrating radar has been used extensively for NDE testing in many applications (e.g., oil and coal exploration, location of subsurface utilities, and detailed surveys to locate small voids or cracks in pavement). Ground-penetrating radar is also being used to determine the thickness and moisture content of asphalt pavement. Geophysical Survey Systems, Inc. (GSSI) has developed a high-speed radar for pavement structure evaluation including mapping asphalt thickness overlays, locating voids and large-scale structural problems, and checking the quality of the thickness of new asphalt concrete pavements. To date, GSSI has not developed a GPR unit that determines asphalt pavement density during the compaction process in real time. Computer programs exist which determine the density and water (or other fluid) content of the various layers within a multilayer system, using conventional GPR to obtain digitized images of a reflected radar signal from a multilayer pavement system. Such a system is not focused on determining the density of asphalt pavement during the rolling process; but, such correlations could theoretically be developed. Other prior art GPR systems include micropower impulse radar (MIR).
Another prior art method of testing asphalt densities involves the use of capacitance energy dissipation (CED) equipment. The CED method measures actual air voids based on the decay rate of energy stored in the asphalt segment when pulsed and compared to the decay rate of a reference capacitor with a known decay rate. CED involves placing a plate on the asphalt and charging it up while looking at the rate of dissipation of the charge. Various "correction factors" are required with this approach. The CED approach also requires stationary contact with the pavement surface. In addition, a temperature correction must be used since the results from a CED device vary depending on the temperature. Also since the plate makes contact with the pavement, it will have to be periodically cleaned.
One major problem with all prior art density testing techniques involves errors from variations in temperature, binder mix, and the aggregate. In other words, a prior art gauge may be calibrated for one binder for one temperature, etc. but then may be used to test asphalt using different binders, aggregates or at different temperatures. As a result, the user must recalibrate the gauge to correct for these variables to ensure an accurate reading.
The following references describe the prior art in detail and are incorporated by reference herein:
Al-Qadi, I. L., Ghodhaonkar, D. K., and Varadan, V, K., 1991. Effect Of Moisture On Asphaltic Concrete At Microwave Frequencies, IEEE Trans. Geoscience & Remote Sensing, Vol. 29, 710-717. PA0 Alongi, A. V., and A. J. Alongi. Subsurface Inspection Radar. U.S. Pat. No. 4,698,634, Oct. 6, 1985. PA0 "America's Highways Accelerating the Search for Innovation," Transportation Research Board, National Research Council, Special Report 202, Washington, D.C., Strategic Transportation Research Study: Highways. PA0 Anderson, D. A., D. W. Christensen, H. U. Bahia, R. Dongre, M. G. Sharma, C. E. Antle, and J. Button. Binder Characterization and Evaluation Volume 3: Physical Characterization. Strategic Highway Research Program, SHRP-A-369, National Research Council, Washington, D.C. 1994. PA0 Burati, J. L. Jr., and G. B. Elzoghbi. Correlation of Nuclear Density Results with Core Densities. Transportation Research Board, Transportation Research Record 1126, 1987. PA0 Coon, J. B., and C. J. Schafers. Earth Probing Radar System. Conoco Inc., U.S. Pat. No. 4,430,653, Feb. 7, 1984. PA0 Davis, J. L., Rossiter, J. R., Mesher, D. E., and Dawley, C. B., 1994. Quantitative Measurement Of Pavement Structures Using Radar. Proceedings of the 5th Annual Conference on Ground Penetrating Radar, June 12-16, Kitchener, Ontario, Canada, pp. 319-334. PA0 Fowler, J. C., L. A. Rubin, and W. L. Still. Synthetic Pulse Radar including a Microprocessor Based Controller. Ensco, Inc., U.S. Pat. No. 4,218,678, Aug. 19, 1980. PA0 Goodman, D. "Ground-Penetrating Radar Simulation In Engineering And Archaeology," Geophysics, Vol. 59, No. 2, February, 1994, pp. 224-232. PA0 GSSI High Speed Radar Pavement Structure Evaluation System. SIR-10H Data Acquisition Control Unit. Brochure, Geophysical Survey System, Inc., North Salem, N.H., Oct. 30, 1995. PA0 Grigas, J. Microwave Dielectric Spectroscopy of Ferroelectrics and Related Materials. Gordon & Breach Publishers, Inc. 1996. PA0 Han, H. C., and C. S. Wang, Microwave Imaging In Inhomogeneous Materials, Proc. Prog. In Electromag. Res. Sym., July 24,28, Seattle, Wash., 1995. PA0 Han, H. C., and C. S. Wang, Coherent microwave imaging for buried objects, Review in Progress in Quantitative Nondestructive Evaluation, 14, 607-613, Plenum, New York, 1995. PA0 Han, H. C., and C. S. Wang, Microwave imaging in inhomogeneous media, Advanced Microwave and Millimeter Wave Detectors, Eds. Udpa and Han, Proc. SPIE 2275, 226-230, 1994. PA0 Imai, Tsuneo, T. Sakayama, and T. Kanemori. "Use Of Ground-Probing Radar And Resistivity Surveys For Archaeological Investigations," Geophysics, Vol. 52, No. 2, February, 1987, pp. 137-150. PA0 Kaufman, A. A., and J. D. McNeill. Signal Processing Apparatus for Frequency Domain Geophysical Electromagnetic Surveying System. Geonics Limited, U.S. Pat. No. 4,544,892, Oct. 1, 1985. PA0 Khan, R. B. Gamberg, D. Power, J. Walsh, B. Dawe, W. Pearson, and D. Millan. Taret Detection And Tracking With A High Frequency Ground Wave Radar. IEEE Journal of Oceanic Engineering 19(4), October 1994, 540-548. PA0 Kovas, J. E. Survey Applications Of Ground-Penetrating Radar. Surveying and Land Information Systems 51(3), 1991, 144-148. PA0 Lytton, R. L. (1995a) System Identification and Analysis of Subsurface Radar Signals. The Texas A&M University System, U.S. Pat. No. 5,384,715, Jan. 24, 1995. PA0 Parra, J. O., T. E. Own, and B. M. Duff. Method and Apparatus for Detecting Subsurface Anomalies. Southwest Research Institute, U.S. Pat. No. 4,835,474, May 30, 1989. PA0 Roberts, F. L., P. S. Kandhal, E. R. Brown, D. Y. Lee, and T. W. Kennedy. Hot Mix Asphalt Materials, Mixture Design, and Construction. NAPA Research and Education Foundation. March 1994. PA0 Schroeder, D. Method and Apparatus for Detecting and Measuring Inclusions in Subsoil. U.S. Pat. No. 4,245,191, Jan. 1, 1981. PA0 Thomas, B. J. Method for Using Seismic Data Acquisition Technology for Acquisition of Ground Penetrating Radar Data. Conoco Inc., U.S. Pat. No. 5,113,192, May 12, 1992. PA0 Troxler 3430 Road/Reader Brochure. Troxler Electronic Laboratories, Inc. Research Triangle Park, North Carolina, 1995. PA0 Vaughan, C. J. "Ground-penetrating radar surveys in archaeological investigations," Geophysics, Vol. 51, No. 3, March, 1986, pp. 595-604. PA0 Von Hippel, A., editor. Dielectric Materials and Applications. Published jointly by The Technology Press of M.I.T. and John Wiley & Sons, New York, 1954, 356.
Measuring density changes of hot mix asphalt pavement throughout the compaction process depends primarily on the physical or chemical properties of the asphalt. Studies have been made to investigate the dielectric properties of asphalt pavements and its constituents. The dielectric properties for several types of organic and inorganic solids, including asphalt, have been determined. Studies have shown that dielectric measurements discriminate between asphalt sources and that dielectric properties are significantly affected by aging. Studies have also found that this method did not discriminate between different asphalts when in the presence of the aggregate since the dielectric constants for the aggregate are approximately one order of magnitude larger than those for the asphalt binders, and therefore the dielectric properties of the aggregate dominate the measured values for the mixes. To date, no research has been performed correlating electromagnetic signals to asphalt pavement density in real time during the compaction process. The present invention develops such correlations as well as tests an innovative density measurement approach that minimizes dependence on the numerous changing parameters of the hot mix asphalt pavement during the rolling process. With the desired correlations developed, the approach of the present invention is faster and more reliable than using conventional GPR techniques, for example.
A need can therefore be seen for a nondestructive fast device that would carefully monitor pavement quality on an in-process, real-time basis. Such a device should measure and simplistically display the parameter that indicates pavement quality; it should also provide feedback to roller operators, enabling them to make corrections during the rolling operation.
Features of the Invention
A general feature of the present invention is the provision of a method and apparatus for sensing asphalt pavement quality in real time which overcomes problems found in the prior art.
A further feature of the present invention is the provision of a method and apparatus for sensing asphalt pavement quality in real time using a differential approach during the asphalt rolling operation.
Further features, objects and advantages of the present invention include:
A method and apparatus for sensing asphalt pavement quality in real time using dual microwave signal sensors.
A method and apparatus for sensing asphalt pavement quality in real time using dual continuous wave sensors.
A method and apparatus for sensing asphalt pavement quality in real time using dual ground penetrating radar sensors.
A method and apparatus for sensing asphalt pavement quality in real time using dual nuclear gauge sensors.
A method and apparatus for sensing asphalt pavement quality in real time using dual capacitance energy dissipation (CED) devices.
A method and apparatus for sensing asphalt pavement quality in real time using dual laser acoustic sensors.
A method and apparatus for sensing asphalt pavement quality in real time which uses dual sensors and monitors the difference in the variance between the two sensors.
A method and apparatus for sensing asphalt pavement quality in real time which can be used without knowing the temperature, binder type, and aggregate type used in the asphalt.
A method and apparatus for sensing asphalt pavement quality in real time capable of determining the asphalt density in real time during the compaction process.
A method and apparatus for sensing asphalt pavement quality in real time which makes no contact with the surface of the asphalt.
A method and apparatus for sensing asphalt pavement quality in real time which is faster and more reliable than existing density measurement techniques.
A method and apparatus for sensing asphalt pavement quality in real time which provides benefits such as reduced construction and maintenance costs, improved pavement quality, faster paving times, and increased safety of motorists on asphalt roads and highways.
A method and apparatus involving an asphalt pavement quality sensor for real-time control of paving density which minimizes the frequent disparities between contractor field lab results and agency lab results.
These as well as other features, objects and advantages of the present invention will become apparent from the following specification and claims.